1. Field of the Invention
The present invention relates to a process and apparatus for producing foamed bodies of thermoplastic synthetic resin by extruding a foamable thermoplastic synthetic resin from a die provided in front of an extruder, foaming the extrudate to a desired shape in a reduced pressure zone and delivering the foamed body into the atmosphere continuously.
2. Description of the Prior Art
Foamed bodies of thermoplastic synthetic resin are produced generally by injecting a blowing agent into a molten resin within an extruder, cooling and kneading the mixture in the extruder to prepare a uniform composition having a melt viscosity suitable for foaming and extruding the composition from a die into the atmosphere to foam the molten thermoplastic resin by virtue of the difference between the vapor pressure of the blowing agent and the atmospheric pressure.
Foamed bodies prepared by such a conventional process usually have an expansion ratio of about 1.5 to about 38 times. Those foamed or expanded to ratios higher than the above-mentioned range are almost of little or no commercial value, for example, because they have an open cellular structure, are uneven in cell diameter and are prone to shrinkage, warping or the like. The conventional process has another drawback in that the use of an increased amount of blowing agent entails a higher material cost if it is desired to obtain products with high expansion ratios even within the above range, for example, with expansion ratios of about 36 to about 38 times.
To overcome these problems, attempts have been made to improve the expansion efficiency by extruding a foamable synthetic resin into a reduced pressure zone as proposed in Japanese Patent Examined Publication No. 18779/1961 and Japanese Published Unexamined Publication No. 2045/1980 and U.S. Pat. No. 4,199,310. However, I have made detailed investigations into these proposals and found that they have the following drawbacks.
Japanese Patent Examined Publication No. 18779/1961 proposes a technique which employs an expansion molding apparatus disposed in front of the die of an extruder, having a space conforming in cross section to the shape of the product, and formed in its inner surface with a large number of ports in communication with a gas discharge device. With this technique, a molten synthetic resin having a blowing agent incorporated therein is continuously extruded into the apparatus and subjected to expansion molding while being passed through the interior of the apparatus which is maintained at a reduced pressure by suction acting through the large number of ports. With this process, the maintenance of the reduced pressure within the expansion molding apparatus is dependent on the state of intimate contact between the foamed resin and the wall surface of the apparatus. However, if it is attempted to produce foamed bodies of different specifications involving different specific gravities with use of expansion molding apparatus having an identical configuration, the optimum values of extrusion molding conditions, such as the amount of blowing agent, temperature of resin, reduced pressure within the die, etc., differ from specification to specification, with the result that the resin tends to foam in varying modes to alter the state of intimate contact between the foamed resin and the wall surface of the apparatus until the operation stabilizes under proper conditions.
This varies the internal reduced pressure of the apparatus, giving rise to variations in the quality of the product. Furthermore, the technique not infrequently involves the need to selectively use an expansion molding apparatus which is designed and fabricated in accordance with the grade or specifications of a particular product every time such specifications are changed. Thus the proposed technique is very disadvantageous economically as an industrial technique.
The invention disclosed in Japanese Published Unexamined Publication No. 2045/1980 relates to a technique which employs a sizing apparatus comprising a sizing cylinder defining a reduced pressure zone, a sealing member provided at the forward end of the cylinder for closing the zone and a cooling water channel externally surrounding the cylinder. With this technique, a prefoamed body of foamable thermoplastic resin composition is extruded into the sizing cylinder, further expanded in the reduced pressure zone within the cylinder and hardened by cooling.
The invention nevertheless has basic technical defects in the method of cooling the foamed body, and in the expansion molding process when producing an article of hollow cross section. The foamed body cooling method is dependent on the transfer of radiant heat resulting from the difference between the temperature of the foamed body and the lower temperature of inner wall surface of the sizing cylinder which is obtained by passing cooling water through the jacket around the apparatus. Consequently the method is very low in cooling efficiency and has the disadvantage that the sizing apparatus has an unjustifiably increased length and requires a larger area for installation because the length of the apparatus is determined by the cooling effect which is irrelevant to the essential feature of the technique, i.e., the production and maintenance of the reduced pressure. Further when producing a foamed body of hollow cross section, the inside hollow portion of the body is subjected to the atmospheric pressure at all times owing to the characteristics of the means used in the technique for sealing off the reduced pressure zone. This entails the drawback that the product differs in cell diameter between the inside of the product and the outside thereof. The process has another drawback in that the outside shape of the product is dependent largely on the atmospheric pressure to which the product is exposed at the inside.
The technique disclosed in U.S. Pat. No. 4,199,310 resides essentially in the maintenance of reduced pressure within an expansion molding appartus by a water seal. As compared with conventional processes, the disclosed technique has the drawback of necessitating a greatly increased area for the installation of the apparatus and requiring a take-up unit of much larger capacity as installed in a vacuum. The above drawback is attributable chiefly to the fact that since the foamed body is held submerged at all times during production, the buoyancy of the body increases greatly with an increase in the size or expansion ratio of the body desired, and also to the fact that there must be an approximately 10-meter difference in elevation between the extruder and the finishing means in order to obtain a required degree of vacuum.